Low star formation efficiency in typical galaxies at z=5-6

Abstract: Using the VLA and ALMA, we have obtained CO(2-1), [C II], [N II] line emission and multiple dust continuum measurements in a sample of "normal" galaxies at $z=5-6$. We report the highest redshift detection of low-$J$ CO emission from a Lyman Break Galaxy, at $z\sim5.7$. The CO line luminosity implies a massive molecular gas reservoir of $(1.3\pm0.3)(\alpha_{\rm CO}/4.5\,M_\odot$ (K km s$^{-1}$ pc$^2)^{-1})\times10^{11}\,M_\odot$, suggesting low star formation efficiency, with a gas depletion timescale of order $\sim$1 Gyr. This efficiency is much lower than traditionally observed in $z\gtrsim5$ starbursts, indicating that star forming conditions in Main Sequence galaxies at $z\sim6$ may be comparable to those of normal galaxies probed up to $z\sim3$ to-date, but with rising gas fractions across the entire redshift range. We also obtain a deep CO upper limit for a Main Sequence galaxy at $z\sim5.3$ with $\sim3$ times lower SFR, perhaps implying a high $\alpha_{\rm CO}$ conversion factor, as typically found in low metallicity galaxies. For a sample including both CO targets, we also find faint [N II] 205$\,\mu$m emission relative to [C II] in all but the most IR-luminous "normal" galaxies at $z=5-6$, implying more intense or harder radiation fields in the ionized gas relative to lower redshift. These radiation properties suggest that low metallicity may be common in typical $\sim$10$^{10}\,M_\odot$ galaxies at $z=5-6$. While a fraction of Main Sequence star formation in the first billion years may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.